Island antenna for installation on aircraft



April 16, 1963 A. ALFORD ISLAND ANTENNA FOR INSTALLATION 0N AIRCRAFT 2SheetsSheet 1 Original Filed Aug. 13, 1954 INVEN TOR. Andrew A/fi y WApril 16, 1963 A. ALFORD ISLAND ANTENNA FOR INSTALLATION 0N AIRCRAFT 2Sheets-Sheet 2 Original Filed Aug. 13, 1954 INVENTOR. Ana raw Al #4United States Patent 3,086,204 ISLAND ANTENNA FOR INSTALLATION ONAIRCRAFT Andrew Alford, Winchester, Mass. (299 Atlantic St., Boston 10,Mass.) Continuation of application Ser. No. 449,717, Aug. 13, 1954', nowabandoned. This application Nov. 27, 1959, Ser. No. 855,940

5 Claims. (Cl. 343-708) The present invention relates to an antennawhich may be installed in a flat surface structure such as in the wingand stabilizer surfaces of an aircraft. The antenna of the type of thepresent invention may be installed without impairing in any way thestrength, efficiency or operation of the structure of the plane or otherplace where the installation is made. This application is a continuationof copending application Serial No. 449,717 filed August 13, 1954, nowabandoned.

While slot antennas were tried by the applicant for this purpose, theyoffer structural difiiculties because the necessary length of the slot,particularly at the lower frequencies, makes it diflicult to providestructures without cutting a main spar or other element of thestructure. In the present invention a flush type of antenna set in thesurface is provided without any necessity of modifying the structure ofthe main sp-ar or other principal structural members in the stabilizeror wings of the plane.

In the present invention a hole is cut in the skin or surface of thestabilizer or other element in which the antenna is to be mounted, andin this hole which may be rectangular in shape, or of any other suitableshape, is mounted and rivetted a smaller rectangular metal sheet in sucha way that an insulated gap of uniform width separates the skin of thestabilizer or other structure which is of metal from the rectangularmetal sheet formed as an island member within the hole structure. Apiece of fiber or other insulating material may cover the island memherand the gap and provide a smooth covering surface.

Such antenna structure may be fed by a coaxial cable with one conductorof the cable connected to the metal sheet on one side, and the otherconductor of the cable connected to the outer metallic surface oppositethe point of connection of the cable to the metal sheet.

The radiation properties of such an antenna depend upon the dimensionsof the structure of the antenna in terms of width, length and upon thepoint at which the coaxial feeder or other feeder for supplying power isconnected across the gap.

If the island formed by the metallic sheet has a vertical axis ofsymmetry and the feeder connection is made along the vertical axis ofsymmetry, the structure will radiate vertically polarized waves.

If the feeder is connected along a horizontal axis of symmetry, thestructure radiates horizontally polarized waves.

This, however, is a relatively crude statement referring only to theprincipal plane of radiation. It becomes more accurate when theproperties of the structure are so chosen that the overall length of thegap separating the surrounding skin from the island within as measuredfrom the feeder around the gap to the feeder again is between one-halfwavelength and a wavelength.

When the overall lengthof the slot is approximately one-half wavelength,the following behavior is observed. The slot acts something like atransmission line. Along the slot there is a standing wave. The maximumvoltage is developed at a point opposite the point of feed. The voltageacross the gap next to the feeder is relatively low in comparison to thevoltage existing across the slot on the opposite side of the feeder.

3,086,204 Patented Apr. 16, 1963 Under these conditions the portion ofthe slot near the feeder contributes only a small part to the totalradiation. The portion of the slot or gap opposite the feedercontributes the main portion of the radiation. The portions of the slotwhich are perpendicular or run across from the portion of the slot wherethe feeder is connected to the opposite side of the slot, radiate waveswhich are 180 out of phase with each other in a direction normal to theskin. Therefore, these portions of the radiation in general cancel outin the direction normal to the skin. As the radiation of the portion ofthe slot opposite to the feed portion is large compared to that of thefeed portion, the radiation of the portion opposite to the feed portioncomprises t-he principal portion of the radiation of the antenna. Thisanalysis holds true where the overall length of the slot or gap is ahalf wavelength. Where the length of the slot or gap is in theneighborhood of a wavelength, the portion of the slot in the vicinity ofthe feed and the portion of the slot opposite to the feed are in thesame phase in the direction normal to the skin and therefore willradiate additively in this normal direction. The radiation of theportions of the slot connecting the first two main portions are 180 outof phase with each other in the direction normal to the skin andtherefore tend to cancel each other out. In addition the radiation ofpower from these last two mentioned portions of the slot are relativelylow, (1) because the voltage minima occurs at the center of the slots,and (2) because the voltage on opposite sides of the voltage minima arein opposite directions so that neither of these two sections of the slotconnecting the other sections would radiate anything in the direction ofthe normal, even individually.

The conditions which exist when the overall length of the slot isbetween one-half wavelength and a wavelength are intermediate of the twoconditions just considered.

Thus by and large, the principal radiation is always either from thesection of the slot opposite to the section which is fed, or is from thesection which is fed and the section opposite to it, and the antennaitself radiates waves which are polarized primarily in the direction ofthe axis of symmetry which passes through the feeder.

For the reasons which have been set forth above, the antenna of thepresent invention is particularly well suited for installation in avertical stabilizer of an aircraft for providing vertical or horizontalpolarization.

Such antennas for lower frequencies, such as megacycles instead of beinga dimension of 5 ft. in length, may be made as small as 15" to 30" forthe size of the island.

It is further possible with the antenna of the present invention toprovide a construction which does not need a cavity in back of the slot,since the island antenna of the present invention seems to be relativelyinsensitive to the size and shape of the cavity behind it. In fact whentwo island antennas are used, one on each side of the verticalstabilizer, there is no need for any cavity in the back of each antenna.Further the island antenna of the present invention is relatively broadband and can be made broad band by relatively direct means ofcompensation.

Without further describing the theory, principles and object of thepresent invention, the invention will be more specifically described inconnection with the drawings attached hereto showing embodiments of theinvention as actually constructed and used.

In the drawings:

FIG. 1 shows an antenna structure of the present invention in a verticalstabilizer connected in such a way as to radiate horizontally polarizedwaves.

FIG. 2. illustrates diagrammatically the construction installed in avertical stabilizer on both faces of the stabilizer for radiatinghorizontally polarized waves.

FIG. 3 is an enlarged diagrammatic representation of FIG. 2, and,

FIG. 4 shows the structure of a balum which may be used in connectionwith FIGS. 2 and 3.

In the structure shown in FIG. 1, 1 indicates the outer metallic skin ofa stabilizer of an aircraft or other metallic surface in which theisland antenna is installed. 2 indicates a metallic island which issurrounded by the skin of the stabilizer. An air gap 3 is formed betweenthe surface 1 and the surface 2. This air gap extends all around theisland 2 and it may be of uniform size. Fiberglass or other insulatingor a non-conductive plate 4 may be rivetted by means of a series ofrivets 5-5 to the surface of the stabilizer and by means of a series ofrivets 66 to the surface of the island. This plate may be attached onthe outer surface of the stabilizer, or it could be attached on theinner surface of the stabilizer whichever is more practical forstructural reasons. The air gap 3 may be filled with insulating materialforming a part or flange of the covering member holding the island 2- inplace.

A concentric feeder 7 having an inner conductor 8 and an outer conductor9 may be used to feed the antenna with the outer conductor connected tothe outer surface of the gap forming the metallic skin of the stabilizerand the inner conductor 3 connected across the gap to the island as forinstance at a point 10.

In the structure shown in FIG. 1, if the wavelength of the gap isapproximately one-half wavelength within the operating range at thefrequency at which the antenna is fed, a maximum will be attained on thevertical side C in the vicinity opposite the feed point at It and aminimum will be presented in the vicinity of the feeder. This means thatmost of the radiation will occur on the side C of the air gap ratherthan on the side A. Therefore, the radiation of the gaps A and C will bepredominantly normal to the skin on the outer surface or stabilizer asused on an airplane and will be horizontally polarized as viewed in FIG.1.

The radiations in the sections B and D of the gap will be in oppositedirections and therefore will tend to cancel one another out.

If the feed, instead of being on the axis Y-Y in FIG. 1 were on the axisX-X, the minimum radiation would be either in sections B or D and theradiations in the sections A and C would be opposed to one another andcancel one another out.

Under these conditions the radiations will be vertically polarizedrather than horizontally polarized as previously set forth.

In FIG. 2 a vertical stabilizer of a plane is shown somewhat inperspective with two island antennas 11 and 12 both fed on one side atsubstantially the mid-point of the upright section of the gap. In theseantennas the chief sections of radiation at a half wavelength, would bein the vertical sections of the gap 13 and 14 and the radiation of bothantennas would be horizontally polarized and radiate horizontallypolarized waves.

In the drawing of FIG. 2 there is shown a balun 15 connected to the feedline. The connection of FIGURE 3 may be used for an omnidirectionalpattern. In this case the impedance presented to the balun is equal tothe impedances of the antennas connected in series.

If the balun is arranged in accordance with the construction of FIG. 4,it presents a capacitive shunt reactance at high frequencies and aninductive reactance at lower frequencies.

The balun in this way improves the impedance spread of the antenna overthe frequency range. A balun sufficient for the desired compensationused in experimental purposes from 108 megacycles to 132 megacycles wasabout 6" long and 2" in diameter, full scale. Furthermore the standingwave ratio without any further connections (such as a transformer) wouldbe below about 4 to 1.

In the structure shown in FIG. 3, there is shown two island antennas as11 and 12 in FIG. 2, each having the same rectangular gap 13 and 14respectively fed by coaxial cables 16 and 17 with the inner conductors18 and 19 connected to the islands 21) and 21 on the inside of the gaps13 and 14 respectively, with compensating capacities in the form ofshunt condensers across the transmission lines between the points offeed P and G. The condensers may be short lengths of coaxialtransmission line with one conductor connected to the respective ones offoils 11 and 12 and the other, to the respective ones of conductingsheets 25 and 21. The broken lines above P in FIG. 3 indicate thelocation of such capacitors.

The balun 15 may be of the type shown in further detail in FIG. 4consisting of outer cylindrical shell 22 with an inner coaxial feed line23 having an inner conductor 24 and a compensating parallel section 25generally of the same size as the outer coaxial conductor '23 andconnected to it at its upper end through a capacitor 26. The upper endsof both sides of the balun may have outer and inner conductor sections27 and 28 providing a balanced feed for antenna sections on both sidesof the stabilizer.

The conductor 25 as indicated in FIG. 4, will at its top end beconnected to the inner conductor 24 of the coaxial cable 23 and thecoaxial cable 27 will have its outer conductor 29 connected to the topplate 30 of the balun, while the outer conductor 31 of the coaxial cable28 will also be tied to the top plate 3! The inner conductor 32 of theouter conductor 28 will go through the top plate 30 and connect to thetop end of the conductor 25 which has no inner conductor in its inside,while the inner conductor 33 of the coaxial cable will connect to theouter conductor 23 within the balun. The balun is fed at its bottomthrough the coaxial cable projecting therefrom, which comprises theouter conductor 23 and the inner conductor 24 extending upward throughthe coaxial cable within the balun at the left.

The antennas on both sides of the stabilizer may be used together forhorizontally or vertically polarized radiation or they may be usedindividually with one connected for horizontally polarized radiation andthe other connected for vertically polarized radiation.

As will be understood from the invention herein described, the antennais of a flush mounted type and will not interfere in any way with theoperation of the plane nor in any way weaken the stabilizer because ofits small size.

What is claimed is:

1. In combination with a conductive airfoil having opposite generallyparallel conductive surfaces in an aircraft, a pair of substantiallyparallelly aligned antennas one mounted in each surface of said airfoil,said antennas having aligned rectangular conductive sheets, meansforming a dielectric gap of uniform width about each sheet in the planeof the respective airfoil surface having a length dimension of between ahalf and a whole wave length in the range of the operating frequency, apair of coaxial lines one each feeding said antenna across said gap atcorresponding positions in a plane dividing said rectangular sheets intocongruent rectangles, each of said positions defined by a pair offeedpoints on opposite sides of and immediately adjacent to said gap,whereby an additive radiation field may be obtained in the plane of saidairfoil polarized generally parallel to said plane, a balun having firstand second balanced terminals, an unbalanced terminal and a plurality ofreference terminals with means for maintaining said reference terminalsat a common potential, first and second unbalanced transmission linescoupling a respective one of said antennas between a respective balancedand unbalanced terminal, and a third unbalanced transmission line forexchanging energy with said unbalanced terminal.

2. A device as set forth in claim 1 and further comprising means forestablishing additional shunt capacity across the points of feed of saidantennas.

3. An aircraft antenna operative over a frequency range centered about aprescribed high frequency comprising, a vertical stabilizer on saidaircraft having nearly parallel vertical conducting airfoils, eachformed with equal area opposed rectangular openings, a pair of likerectangular conducting sheets, means including insulating material formaintaining a respective conducting sheet centered within each of saidrectangular openings insulatedly separated from said conducting airfoilsto define a pair of opposed circumferential gaps each surrounding arespective conducting sheet, each of said gaps being symmetrical aboutrespective orthogonal pairs of axes lying in the plane of a respectiveairfoil, a pair of coaxial transmission lines each having an inner andouter conductor, said conductors of a respective line being respectivelyconnected to an associated airfoil and an associated one of saidconducting sheets at points immediately adjacent to and separated by anassociated one of said gaps, said points of connection across one gapbeing directly opposite to said points of connection across the otherand lying on one of said axes, the circumferential length of each gapbeing approximately one-half to one wavelength of energy within saidfrequency range, and means for exchanging energy with said coaxialtransmission lines so that the potential developed at the feed point ofone conducting sheet is oppositely sensed with respect to that developedat the other conducting sheet feed point referenced to the potential onsaid conducting airfoil.

4. A radiating system operative over a relatively wide frequency rangeabout a center high frequency comprising, a pair of closely-spacedopposed generally parallel conducting surfaces substantially maintainedat a common potential and formed with congruent openings therein, aconducting plate within each of said openings generally coplanar withand insulatedly separated from a respective one of said conductingsurfaces to define first and second congruent perimetrical insulatinggaps surrounding a common axis, the length of each of said gaps being ahalf wavelength to a wavelength for frequencies within said widefrequency range and means for establishing oppositely sensed potentialswith respect to said comm-on potential at opposed points on saidconducting plates immediately adjacent to a respective gap to establisha standing Wave pattern a half to a full wavelength long for frequencieswithin said frequency range in both said gaps and provide substantiallyomnidirectional radiation characteristics about an axis generallyparallel to said surfaces and perpendicular to said common axis.

5. A radiating system in accordance with claim 4 wherein said means forestablishing comprises, first and second coaxial transmission lines eachhaving inner and outer conductors, a balun having first and secondbalanced terminals, an unbalanced terminal and a plurality of referenceterminals, the first and second transmission line inner conductorsrespectively coupling said first and second balanced terminals torespective ones of said opposed points on the respective conductingplates, the first and second transmission line outer conductorsrespectively coupling reference terminals to respective ones of theremaining opposed points.

References Cited in the file of this patent UNITED STATES PATENTS2,127,088 Percival et a1 Aug. 16, 1938 2,781,512 Robinson Feb. 12, 19572,908,000 Robinson Oct. 6, 1959 FOREIGN PATENTS 651,806 Great BritainApr. 11, 1951 655,045 Great Britain July 11, 1951 668,223 Great BritainMar. 12, 1952 1,012,833 France July 17, 1952

1. IN COMBINATION WITH A CONDUCTIVE AIRFOIL HAVING OPPOSITE GENERALLYPARALLEL CONDUCTIVE SURFACES IN AN AIRCRAFT, A PAIR OF SUBSTANTIALLYPARALLELLY ALIGNED ANTENNAS ONE MOUNTED IN EACH SURFACE OF SAID AIRFOIL,SAID ANTENNAS HAVING ALIGNED RECTANGULAR CONDUCTIVE SHEETS, MEANSFORMING A DIELECTRIC GAP OF UNIFORM WIDTH ABOUT EACH SHEET IN THE PLANEOF THE RESPECTIVE AIRFOIL SURFACE HAVING A LENGTH DIMENSION OF BETWEEN AHALF AND A WHOLE WAVE LENGTH IN THE RANGE OF THE OPERATING FREQUENCY, APAIR OF COAXIAL LINES ONE EACH FEEDING SAID ANTENNA ACROSS SAID GAP ATCORRESPONDING POSITIONS IN A PLANE DIVIDING SAID RECTANGULAR SHEETS INTOCONGRUENT RECTANGLES, EACH OF SAID POSITIONS DEFINED BY A PAIR OFFEEDPOINTS ON OPPOSITE SIDES OF AND IMMEDIATELY ADJACENT TO SAID GAP,WHEREBY AN ADDITIVE RADIATION FIELD MAY BE OBTAINED IN THE PLANE OF SAIDAIRFOIL POLARIZED GENERALLY PARALLEL TO SAID PLANE, A BALUN HAVING FIRSTAND SECOND BALANCED TERMINALS, AN UNBALANCED TERMINAL AND A PLURALITY OFREFERENCE TERMINALS WITH MEANS FOR MAINTAINING SAID REFERENCE TERMINALSAT A COMMON POTENTIAL, FIRST AND SECOND UNBALANCED TRANSMISSION LINESCOUPLING A RESPECTIVE ONE OF SAID ANTENNAS BETWEEN A RESPECTIVE BALANCEDAND UNBALANCED TERMINAL, AND A THIRD UNBALANCED TRANSMISSION LINE FOREXCHANGING ENERGY WITH SAID UNBALANCED TERMINAL.